In the use of geothermal brines for recovering the energy values thereof for use, for example, in power plants, and in recovery of combustion gas from geopressurized fluids, a number of problems arise. One of these problems is scale formation which occurs when the brine is flashed and/or drops in temperature, leading to the deposition of scale on adjacent surfaces. Scaling sometimes becomes so extreme that plant operation must be shut down after a relatively short operating time for maintenance to remove the scale so that the plant can be placed in operation once again as soon as possible.
Other problems which arise include those related to the presence of suspended solids in spent brine which are to be directed back into the ground, such as into a reinjection well for disposal or other purposes. Because of the presence of solids in the spent brine, scaling and plugging of injection pumps, associated equipment and the subterranean formation occur which requires interruption of operation to remove the scale to permit reinjection operations to continue. This interruption may even require the drilling of a new injection well.
In hydrothermal systems, brine in equilibrium with a downhole environment is subjected to a change in thermodynamic conditions as the brine travels up a wellbore and through fluid lines and into plant components. As a result, dissolved gases are released, the brine pH and chemical balance change, and solids in the brine can precipitate to form scale. The problem is further complicated by the progressive changes in temperature and composition as the brine moves through the plant and back into the ground, depositing scales of varying compositions at various locations in its transmit through the plant and reinjection well. The normal flash process produces spent brine which is supersaturated with respect to solids, such as silica, metal sulfides and metal sulfates or carbonate compounds. The precipitation of these compounds in plant components and in an injection well along with plugging of the subterranean formations makes it imperative that a solution be found for the problem of scale control.
Conventionally, three methods have been tested and used for the control of scaling in geothermal brines, namely scale retardation or inhibition, chemical and mechanical removal of scale, and the use of redundant equipment. However, none of these techniques has been practiced in which energy and mineral values are simultaneously recovered from the brine. The performance of these conventional techniques is time-consuming and relatively expensive, thereby giving rise to the realization of the need for improvements in the handling of geothermal brines so as to avoid the problems of scale formation while obtaining the greatest amount of energy and mineral recovery from the brines.
For protecting injection wells against scale formation, the reactor-clarifier unit is used. Such a unit operates on the principal of solids contact clarification and not on crystal growth. This requires a reactor-clarifier unit separate from the plant equipment which recovers energy values from geothermal fluid, thereby increasing equipment, labor and operating costs. However, the reactor-clarifier does not protect the geothermal power plant from scaling problems as the reactor-clarifier is located downstream of the power plant.
Another aspect of the handling of geothermal brines is the crystallization of the solid fractions of the brines to extract the mineral values therefrom. This is achieved by crystallization techniques which generally have been known for well over 100 years. However, these techniques have not been associated with the simultaneous extraction or recovery of energy values from the geothermal brine while preventing substantial scale formation. Also crystallization has occured in equipment separate from the plant and equipment used for flashing geothermal brines to recover energy values from the brine.
Two steps are involved in the precipitation of crystal matter from a solution, i.e., the crystals must first form and then grow. The formation of a new solid phase in the solution or in the solution by itself is called nucleation. The increase in size of a crystal with a layer-by-layer addition of solute is called growth. Both nucleation and crystal growth have supersaturation as a common driving force. Unless a solution is supersaturated, crystals therein can neither form nor grow. The problem then is to provide equipment in a crystallizer which promotes nucleation and crystal growth. In the past, this has required that clarification equipment be separate from the flash equipment used for recovering energy from the brines. Thus, because of the expense and consumption of time in the operation of conventional reactor-clarifiers, a need has arisen for improvements in the recovery of mineral values in the form of crystals from the solid fractions of brines without the need for separate clarification equipment.
While there are a number of patents in geothermal energy recovery, they all relate to 2-phase processing instead of 3-phase processing of geothermal fluids.